In EPROM's, a "one" is defined as the state in which the floating gate memory cell conducts at a low threshold voltage Vt, that is, with little or no voltage on the control gate. The source-drain path of a floating gate memory cell conducts at a relatively low threshold voltage Vt if the cell is "erased", that is, there are no excess electrons in the floating gate. A "zero" is the state in which the threshold voltage Vt of the memory cell is raised (programmed) such that the cell does not conduct under normal operating conditions. The source-drain path conducts at a relatively high threshold voltage Vt if the cell is "programmed", that is, there are a sufficient number of electrons trapped on the floating gate to deplete the number of conductors in the source-drain path.
The margin between the threshold voltages Vt of erased memory cells and of programmed memory cells must be such that erased cells have threshold voltages Vt above a reference value and programmed cells have threshold voltages Vt below a reference accomplished by comparing the selected memory cell to a reference cell in such a way that the reference cell defines a condition, current or voltage, that resides between the "one" and "zero" state of the selected bit. Both the reference cell and the selected cell have the same voltage Vcc applied to their control gates during read operation. The comparison to determine whether a "one" or a "zero" is stored on the selected cell is performed by a sense amplifier.
One previous method for improving the sensing capability of sense amplifiers includes changing the width of the load resistor/transistor in the sense amplifier. In the previous method, the difference between the current in the reference memory cell and in the selected memory cell is the same throughout the range from low control gate voltage Vcc to high control gate voltage Vcc.
What is needed is a method of improving the sensing capability of the sense amplifier circuit at low values of control gate voltage Vcc without significantly degrading the sensing capability at high values of control gate voltage Vcc.